1. Field of the Invention
This invention relates to an optical information recording medium suitable for optical writing recording by laser, etc., more particularly to an improved optical information recording medium which can be used for optical disc and optical card.
2. Related Background Art
Generally speaking, an optical disc or optical card can memory a high density information with an optically detectable small, for example, about 1.mu. pit formed on a thin recording layer provided on a substrate being in the track form of a spiral, circular or linear shape. For writing information in such disc or card, converged laser is scanned on the surface of a laser sensitive layer, whereby only the surface irradiated with the laser beam forms pits, which pits are formed in the form of spiral, circular or linear tracks. The laser sensitive layer can form optically detectable pits through absorption of laser energy. For example, according to the heat mode recording system, the laser sensitive layer absorbs heat energy, and can form a small concavity (pit) at that site through vaporization or melting. Also, according to another heat mode recording system, through absorption of the laser energy irradiated, a pit having an optically detectable density difference can be formed at that site.
The information recorded on such optical disc and optical card is detected by scanning laser beam along the track and reading the optical change at the portion where a pit is formed and the portion where no pit is formed. For example, in the case of an optical disc or optical card, laser beam is scanned along the track, and the energy reflected by the disc or the card is monitored by a photodetector. At the portion where a pit is formed, reflection of laser becomes lower and the output from the photodetector becomes smaller. On the other hand, at the portion where no pit is formed, reflection is sufficiently effected to make the output from the photodetector greater.
As an optical information recording medium to be used for such optical disc and optical card, there have been hitherto proposed those using primarily inorganic materials, for example, a metal thin film such as aluminum, gold vapor deposited film, etc., bismuth thin film, tellurium oxide thin film, chalcogenide type amorphous glass film, etc. These thin films have drawbacks such that storability is poor, decomposition ability is low, recording density is low, the cost is high, etc.
Also, it has been recently proposed to use an organic dye thin film which can undergo change in physical properties with a light of relatively longer wavelength for the recording layer. The organic dye thin film can remove the above drawbacks, but an organic dye having absorption characteristic on the longer wavelength side will generally involve a problem in that stability is low to heat and light. Including these points, the organic coloring matter to be used in recording material is required to have the following properties, namely:
1. to have no toxicity;
2. to have an absorption at around 800 nm, with a great coefficient of absorption;
3. to have good solubility in an organic solvent;
4. to have a great reflectance at around 800 nm in a thin film state;
5. to be crystallized with difficulty in a thin film state;
6. to be stable to UV-ray and visible light;
7. to be thermally stable;
8. to be stable to humidity;
9. to be synthesized with ease, etc.
The advantages of organic dye as compared with inorganic compounds which have been used in the prior art may include no toxicity of item 1. and good solubility of item 3., but there is the problem that it is readily crystallized in a thin film state, while having sufficient reflectance.
Reflectance is necessary for scanning of laser on the track with good precision when monitored with a photodetector, but absorbance, reflectance of the organic film will be lowered to great extent if crystallization occurs, whereby pit can be formed with difficulty to make recording impossible.